Schemes for forming barrier layers for copper in interconnect structures

ABSTRACT

A method of forming a semiconductor structure includes providing a substrate; forming a low-k dielectric layer over the substrate; embedding a conductive wiring into the low-k dielectric layer; and thermal soaking the conductive wiring in a carbon-containing silane-based chemical to form a barrier layer on the conductive wiring. A lining barrier layer is formed in the opening for embedding the conductive wiring. The lining barrier layer may comprise same materials as the barrier layer, and the lining barrier layer may be recessed before forming the barrier layer and may contain a metal that can be silicided.

TECHNICAL FIELD

This invention is related generally to integrated circuits, and more particularly to the structure and methods of interconnect structures in integrated circuits, and even more particularly to the formation of barrier layers on copper features.

BACKGROUND

A conventional integrated circuit contains a plurality of patterns of metal lines separated by inter-wiring spacings and a plurality of interconnect lines, such as bus lines, bit lines, word lines and logic interconnect lines. Typically, the metal patterns of vertically spaced metallization layers are electrically interconnected by vias. Metal lines formed in trench-like openings typically extend substantially parallel to the semiconductor substrate. Semiconductor devices of such type, according to current technology, may comprise eight or more levels of metallization layers to satisfy device geometry and micro-miniaturization requirements.

A commonly used method for forming metal lines and vias is known as “damascene.” Generally, this process involves forming an opening in the dielectric interlayer, which separates the vertically spaced metallization layers. The opening is typically formed using conventional lithographic and etching techniques. After an opening is formed, the opening is filled with copper or copper alloys to form a via or a trench. Excess metal material on the surface of the dielectric interlayer is then removed by chemical mechanical polishing (CMP).

Copper has replaced aluminum to form metal lines because of its lower resistivity. However, copper suffers from electro-migration (EM) and stress-migration (SM) reliability issues as geometries continue to shrink and current densities increase.

FIG. 1 illustrates a cross-sectional view of a conventional interconnect structure 1 formed using damascene processes. Metal lines 2 and 4, which are typically formed of copper or copper alloys, are interconnected by via 10. Inter-metal-dielectric (IMD) 8 separates the two layers where metal lines 2 and 4 are located. Etch stop layer (ESL) 5 is formed on copper line 2. Diffusion barrier layers 12 and 14, which typically comprise Ta or TaN, are formed to prevent copper from diffusing into surrounding materials. ESL 5 typically has a higher dielectric constant (k value) than low-k dielectric layer 6 and IMD 8. As a result, the parasitic capacitances between the metal lines are undesirably increased.

FIG. 2 illustrates an alternative interconnect structure 3. Metal cap 16 is formed on copper line 2. Metal cap 16 is typically formed of materials not prone to electro-migration and stress-migration. This layer improves the reliability of the interconnect structure by reducing copper surface migration. It has been found that under stressed conditions, the mean time to failure (MTTF) of interconnect structure 3 is significantly longer than that of interconnection structure 1. With metal cap 16, the stress-induced void formation is also significantly reduced. Additionally, the parasitic capacitances are also reduced.

Since metal cap 16 is typically formed only on copper line 2, weak points exist at the interface of metal cap 16 and diffusion barrier layer 14, and copper may still diffuse out from these weak points.

Alternatively, metal cap 16 may be formed by soaking copper in silane (SiH₄) in a thermal and non-plasma ambient. Copper silicide is thus formed on the surface of copper line 2. A drawback of such a scheme is that during silane soaking, silicon in the silane will diffuse deep into copper line 2, causing copper silicide's formation deeply in copper line 2. As a result, the resistivity of copper line 2 is increased. The problem becomes worse when advanced technologies are used to form integrated circuits, and thus the thickness of copper line 2 is reduced.

The conventional schemes for forming cap layers have advantageous features and disadvantageous features, and thus may be used accordingly for different design requirements. To satisfy different design requirements and improve the reliability of integrated circuits, more methods for forming cap layers on copper lines are needed.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a method of forming a semiconductor structure includes providing a substrate; forming a low-k dielectric layer over the substrate; embedding a conductive wiring into the low-k dielectric layer; and thermal soaking the conductive wiring in a carbon-containing silane-based chemical to form a barrier layer on the conductive wiring.

In accordance with another aspect of the present invention, a method of forming a semiconductor structure includes providing a substrate; forming a low-k dielectric layer over the substrate; forming an opening extending from a top surface of the low-k dielectric layer into the low-k dielectric layer; forming a first barrier layer lining the opening; embedding a conductive wiring into a remaining portion of the opening; recessing a top edge of the first barrier layer to form a recess, wherein portions of sidewalls of the conductive wiring are exposed; and forming a second barrier layer covering a top surface and exposed sidewalls of the conductive wiring.

In accordance with yet another aspect of the present invention, a method of forming a semiconductor structure includes providing a substrate; forming a low-k dielectric layer over the substrate; forming an opening extending from a top surface of the low-k dielectric layer into the low-k dielectric layer; forming a first barrier layer lining the opening; embedding a conductive wiring into a remaining portion of the opening; and forming a second barrier layer covering exposed portions of the conductive wiring, wherein the second barrier layer comprises substantially same materials as the first barrier layer, and wherein the second barrier layer is formed on the top edges of the first barrier layer.

In accordance with yet another aspect of the present invention, a method of forming a semiconductor structure includes providing a substrate; forming a low-k dielectric layer over the substrate; forming an opening extending from a top surface of the low-k dielectric layer into the low-k dielectric layer; forming a first barrier layer lining the opening, wherein the first barrier layer comprises a metal selected from the group consisting essentially of cobalt, nickel, and combinations thereof; embedding a conductive wiring into a remaining portion of the opening; and thermal soaking a top surface of the conductive wiring in a silane-based chemical to form a second barrier layer on the conductive wiring and the top edges of the first barrier layer.

In accordance with yet another aspect of the present invention, a semiconductor structure includes a substrate; a low-k dielectric layer over the substrate, an opening extending from a top surface of the low-k dielectric layer into the low-k dielectric layer; a first barrier layer lining the opening, wherein top edges of the first barrier layer is recessed from a top surface of the low-k dielectric layer to form a recess; a conductive wiring in a remaining portion of the opening; and a second barrier layer covering a top surface of the conductive wiring and extending into the recess.

In accordance with yet another aspect of the present invention, a semiconductor structure includes a substrate; a low-k dielectric layer over the substrate; an opening extending from a top surface of the low-k dielectric layer into the low-k dielectric layer; a first barrier layer lining the opening; a conductive wiring in a remaining portion of the opening; and a second barrier layer on the conductive wiring and top edges of the first barrier layer, wherein portions of the second barrier layer directly on the top edges of the first barrier layer comprise a silicide formed from the first barrier layer.

In accordance with yet another aspect of the present invention, a semiconductor structure includes a substrate; a low-k dielectric layer over the substrate; an opening extending from a top surface of the low-k dielectric layer into the low-k dielectric layer; a first barrier layer lining the opening; a conductive wiring in a remaining portion of the opening; and a second barrier layer on the conductive wiring, wherein the second barrier layer comprises substantially same materials as the first barrier layer, and wherein the second barrier layer is formed on the top edges of the first barrier layer.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a conventional interconnect structure, wherein a copper line is covered by an etch stop layer;

FIG. 2 illustrates a conventional interconnect structure, wherein a copper line is covered by a metal cap;

FIGS. 3 through 5 are cross-sectional views of intermediate stages in the manufacture of a first embodiment of the present invention, wherein a barrier layer is formed on a copper feature by thermal soaking;

FIG. 6 illustrates a cross-sectional view of a second embodiment of the present invention, wherein in addition to silicide formed on a copper line, silicide is also formed on the top edges of a lining barrier layer;

FIGS. 7 and 8 are cross-sectional views of intermediate stages in the manufacture of a third embodiment of the present invention, wherein a barrier layer on a copper line extends into a recess of a lining barrier layer; and

FIG. 9 illustrates a cross-sectional view of a fourth embodiment, wherein a barrier layer formed on a copper line comprises substantially same materials as a lining barrier layer, and thus the barrier layer is also formed on the lining barrier layer.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

The schemes of interconnect structures for integrated circuits and methods of forming the same are provided. The intermediate stages of manufacturing preferred embodiments of the present invention are illustrated. Throughout the various views and illustrative embodiments of the present invention, like reference numbers are used to designate like elements. In the following discussed embodiments, single damascene processes are discussed. One skilled in the art will realize that the teaching is readily available for dual damascene processes.

FIGS. 3 through 5 are cross-sectional views of intermediate stages in the making of a first embodiment of the present invention. FIG. 3 illustrates the formation of opening 26 in dielectric layer 20. As is known in the art, dielectric layer 20 is formed over a substrate (not shown), which may be a single crystalline or a compound semiconductor substrate. Active devices (not shown) such as transistors may be formed on the semiconductor substrate. Opening 26 may be a via opening, which is for forming a via, or a trench, which is for forming a metal line. The width W of opening 26 is preferably less than about 50 nm. In an exemplary embodiment, dielectric layer 20 has a low dielectric constant value (k value), preferably lower than about 3.0, hence is referred to as low-k dielectric layer 20 throughout the description. Low-k dielectric layer 20 may include commonly used low-k dielectric materials such as carbon-containing dielectric materials, and may further contain nitrogen, hydrogen, oxygen, and combinations thereof. Low-k dielectric layer 20 is preferably porous.

FIG. 4 illustrates the formation of diffusion barrier layer 30 lining opening 26 and conductive line 32 in opening 26. Throughout the description, diffusion barrier layer 30 is alternatively referred to as a barrier layer or a lining barrier layer. Barrier layer 30 preferably includes titanium, titanium nitride, tantalum, tantalum nitride, or other alternatives. Barrier layer 30 may be formed using physical vapor deposition (PVD) or one of the chemical vapor deposition (CVD) methods. The thickness of barrier layer 30 may be between about 20 Å and about 200 Å. One skilled in the art will realize, however, that the dimensions recited throughout the description are related to the formation technology used for forming the integrated circuits, and will reduce with the scaling of the formation technology.

The material of conductive line 32 is preferably copper or a copper alloy. Throughout the description, conductive line 32 is alternatively referred to as copper line 32, although it may comprise other conductive materials, such as silver, gold, tungsten, aluminum, and the like. As is known in the art, the steps for forming barrier layer 30 and copper line 32 may include blanket forming barrier layer 30, depositing a thin seed layer of copper or copper alloy, and filling opening 26 with a conductive material, preferably by plating. A chemical mechanical polish (CMP) is then performed to remove excess barrier layer 30 and the conductive material on low-k dielectric layer 20, leaving barrier layer 30 and copper line 32 only in opening 26.

An optional pretreatment is then performed to treat the surface of copper line 32. In the preferred embodiment, the pretreatment includes a hydrogen-based gas environment in a production tool, such as one used for plasma enhanced chemical vapor deposition (PECVD). The hydrogen-based gases preferably include H₂, NH₃, CH₄, and the like. In alternative embodiments, the pretreatment is performed in a nitrogen-based gas environment, which contains nitrogen-containing gases, for example, N₂, NH₃, and the like. The pretreatment has the function of removing oxygen and possibly some chemical contamination from copper line 32.

FIG. 5 illustrates the formation of barrier layer 34, often referred to as metal cap 34, on copper line 32. In one embodiment, the structure as shown in FIG. 4 is thermal soaked in a carbon-containing silane-based soaking gas, such as tri-methyl-silane (SiH(CH₃)₃, also referred to as 3MS, wherein the thermal soaking occurs in an environment (ambient) at an elevated temperature, for example, about 300° C. and higher. Preferably, plasma is not turned on, although in alternative embodiments, plasma may be turned on. In an exemplary embodiment, the temperature of the ambient is between about 150° C. and about 450° C., and the pressure of the soaking gas is between about 10 mtorr and about 1000 mtorr. In alternative embodiments, the carbon-containing silane-based soaking gas may contain methyl-silane (SiH₃(CH₃)₁, also referred to as 1MS), di-methyl-silane (SiH₂(CH₃)₂, also referred to as 2MS), 3MS, and combinations thereof. Tetra-methy-silane (4MS) is generally not preferred. In yet other embodiments, the soaking gas may contain other gases with SiH— bonds. In the thermal environment, the soaking gas reacts with copper to form copper silicide, hence barrier layer 34 is formed on copper line 32. One skilled in the art will perceive that thickness T of barrier layer 34 is related to the temperature and soaking duration. In an exemplary embodiment, the soaking duration is between about 1 second and about 5 minutes. Accordingly, thickness T is between about 50 Å and about 200 Å.

There may be dangling bonds on the surface of barrier layer 34, which may be removed by an additional plasma treatment. Preferably, nitrogen-containing gases, such as NH₃, and/or carbon-containing gases, for example, methyl (CH₃) containing gases, such as C_(x)H_(y), may be used, wherein x and y indicate an atomic ratio of carbon to hydrogen. The treatment will connect the dangling bonds with nitrogen-containing and/or carbon-containing terminals, and thus passivates barrier layer 34.

An advantageous feature of the embodiment shown in FIG. 5 is that during the thermal soaking of copper line 32 in 1MS, 2MS and/or 3MS, the carbon in these materials will prevent silicon diffusing into deep portions of copper line 32. Accordingly, copper silicide is formed only at the surface portion of the copper line 32, avoiding the excess resistance degradation of copper line 32.

FIG. 6 illustrates a cross-sectional view of a second embodiment of the present invention. In this embodiment, the initial structure is similar to the structure as shown in FIG. 4, except that barrier layer 30 comprises a metal that can be silicided. In an exemplary embodiment, barrier layer 30 comprises a cobalt-containing material including CoP, CoB, CoWP, CoWB, or the like. In other embodiments, barrier layer 30 comprises nickel. Barrier layer 34 may be formed using essentially the same thermal soaking method as discussed in the first embodiment. Alternatively, barrier layer 34 is formed by soaking the wafer, on which the barrier layer 30 and copper line 32 are formed, in silane. In an exemplary embodiment, the wafer is placed in an ambient filled with silane gas, wherein the temperature of the ambient is between about 100° C. and about 450° C., and the pressure of the soaking gases is between about 1 mtorr and about 10 torr. Plasma is generally not desired, although it may be turned on.

During the thermal soaking, copper silicide is formed on the surface of copper line 32. At the same time, the metal in diffusion barrier layer 30 also reacts with silicon to form silicide. As a result, region 34 ₁ of barrier layer 34 comprises copper silicide, while regions 34 ₂ comprises the silicide of metals in barrier layer 30. Therefore, the interfaces between barrier layers 30 and 34 are sealed. In an exemplary embodiment, both portions 34 ₁ and 34 ₂ of barrier layer 34 have a thickness of between about 5 Å and about 100 Å. One skilled in the art will realize, however, that the portions 34 ₁ and 34 ₂ may have different thicknesses due to the different silicidation rates between copper and the metals in copper line 32 and barrier layer 30.

FIGS. 7 and 8 illustrate cross-sectional views of intermediate stages of a third embodiment of the present invention. In this embodiment, the initial structure is similar to the structure as shown in FIG. 4, with barrier layer 30 formed of essentially the same material as in the first embodiment. In an exemplary embodiment, barrier layer 30 includes titanium, titanium nitride, tantalum, tantalum nitride, or other alternatives. Alternatively, barrier layer 30 comprises a metal that can be silicided, such as cobalt and/or nickel.

Referring to FIG. 7, after copper line 32 is formed, an etching is performed using an etchant that attacks diffusion barrier layer 30, but not copper line 32 and low-k dielectric layer 20. Recesses 38 are thus formed. In an exemplary embodiment, recesses 38 have a depth D of greater than about 50 Å. Depth D is also preferably greater than about 5 percent of thickness DC of copper line 32.

An optional treatment may be performed to clean surface of copper line 32 using either thermal or plasma treatment, wherein the details of the treatment may be essentially the same as discussed in the first embodiment. After the optional treatment, barrier layer 40 is formed covering top surface and sidewalls of copper line 32, as illustrate in FIG. 8. Diffusion barrier layer 40 may comprise CoP, CoB, CoWP, CoWB, NiWP, CoSnP, NiWB, CuSi, ZrN, NiMoP, and combinations thereof. In an exemplary embodiment, barrier layer 40 is plated using, for example, electrochemical plating. Alternatively, barrier layer 40 is formed by thermal soaking in either a soaking gas containing 1MS, 2MS, 3MS, and combinations thereof, or thermal soaking in silane. The details of thermal soaking have been discussed in the preceding paragraphs, and thus are not repeated herein. Accordingly, barrier layer 40 comprises copper silicide. If diffusion barrier layer 30 contains cobalt and/or nickel, a portion of barrier layer 40 on the top edges of barrier layer 30 may contain cobalt silicide and/or nickel silicide. By forming recesses 38, the weak points between barrier layers 30 and 40 are eliminated.

FIG. 9 illustrates an intermediate stage of a fourth embodiment of the present invention. In this embodiment, the initial structure is similar to the structure as shown in FIG. 4, wherein barrier layer 30 can be formed of any commonly used barrier materials, such as Ta, TaN, Ti, TiN, cobalt-containg material such as CoP, CoB, CoWP, CoWB, nickel-containing material such as NiWP, CoSnP, NiWB, CuSi, ZrN, NiMoP, and combinations thereof. Barrier layer 46 is formed on both copper line 32 and diffusion barrier layer 30. In the preferred embodiment, diffusion barrier layer 46 contains a substantially same material as diffusion barrier layer 30. It is to be noted that the term “substantially same” is a term of art. If barrier layers 30 and 46 have same types of elements in substantially same concentrations, they are considered to be substantially the same. Furthermore, if diffusion barrier layers 30 and 46 have over about 70 percent materials in common, they are considered to be substantially the same, even though they may comprise additional different types of materials. In an exemplary embodiment, barrier layer 46 is formed by electroless plating. Because diffusion barrier layer 30 comprises substantially same materials as diffusion barrier layer 46, diffusion barrier layer 46 can be formed on the top edges of the barrier layer 30 during plating deposition of the diffusion barrier layer 46. Otherwise, diffusion barrier layer 46 will not be formed on the top edges of barrier layer 30. For instance, if diffusion barrier layer 30 is a TaN layer and diffusion barrier layer 46 is a CoWP or a NiWP layer, then CoWP or NiWP will not be formed on top edges of barrier layer 30. The thickness of diffusion barrier layer 46 may be between about 20 Å and about 200 Å.

With barrier layer 46 extending on the top edges of barrier layer 30, a better sealing of copper line 32 is achieved.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

1. A method of forming a semiconductor structure, the method comprising: providing a substrate; forming a low-k dielectric layer over the substrate; embedding a conductive wiring in the low-k dielectric layer; and thermal soaking the conductive wiring in a carbon-containing silane-based chemical to form a barrier layer on the conductive wiring, wherein the thermal soaking occurs in a non-plasma environment, the barrier layer comprising a silicide.
 2. The method of claim 1, wherein the conductive wiring comprises copper.
 3. The method of claim 1, wherein the step of thermal soaking is performed with plasma turned off.
 4. The method of claim 1, wherein the step of thermal soaking is performed at a temperature of between about 150° C. and about 450° C.
 5. The method of claim 1, wherein the carbon-containing silane-based chemical comprises a material selected from the group consisting essentially of SiH₃(CH₃)₁ (1MS), SiH₂(CH₃)₂ (2MS), and combinations thereof.
 6. The method of claim 5, wherein the carbon-containing silane-based chemical is free from Si(CH₃)₄ (4MS).
 7. The method of claim 1, wherein the step of embedding the conductive wiring comprises: forming an opening extending from a top surface of the low-k dielectric layer into the low-k dielectric layer; and forming a lining barrier layer lining the opening, wherein the lining barrier layer comprises a metal selected from the group consisting essentially of cobalt, nickel, and combinations thereof, and wherein during the step of thermal soaking, the metal reacts with the carbon-containing silane-based chemical to form silicide on top edges of the lining barrier layer.
 8. The method of claim 1 further comprising performing a treatment to a surface of the barrier layer using C_(x)H_(y).
 9. A method of forming a semiconductor structure, the method comprising: providing a substrate; forming a low-k dielectric layer over the substrate; forming an opening extending from a top surface of the low-k dielectric layer into the low-k dielectric layer; forming a first barrier layer lining the opening, wherein the first barrier layer comprises a metal selected from the group consisting essentially of cobalt, nickel, and combinations thereof; embedding a conductive wiring in a remaining portion of the opening; and thermal soaking a top surface of the conductive wiring in a silane-based chemical comprising SiH₃(CH₃)₁ (1MS), SiH₂(CH₃)₂ (2MS), and combinations thereof to form a silicide second barrier layer on the conductive wiring and top edges of the first barrier layer.
 10. The method of claim 9 further comprising performing a treatment to a top surface of the silicide second barrier layer using C_(x)H_(y). 